The accurate measurement of the shape and spatial dimensions (size) of small features is vitally important in the semiconductor industry. As the critical dimensions (CDs), such as feature width, shrink towards well below 100 nm, to 10 nm and below, traditional methods of measuring the key parameters required for manufacturing process control are becoming seriously challenged. These techniques include optical scatterometry, also known as optical critical dimension (OCD) metrology, which measures the shape and size of a feature through changes in the amplitude, intensity, and/or polarization of light generally in the infrared (IR) to ultra-violet (UV) range when scattered from a periodic array of the features. Another technique for dimensional analysis is scanning electron microscopy (SEM) and in particular the CD-SEM, which forms a top-down image of the features within the scanning area and can thus provide cross-sectional dimensions of individual features. These two techniques are the most widely used approaches to dimensional analysis in semiconductor manufacturing today.
As the semiconductor industry moves to ever smaller features and high aspect ratio (HAR) features, even the most advanced OCD and CD-SEM tools have issues due both to the small in-plane dimensions of the features to be measured and also to the relative depth of these features. In the case of OCD techniques with small features having such high aspect ratio, there are problems associated with getting the comparatively long-wavelength light into and out of the structures, whereas with CD-SEM only the top of the feature is probed and no information at significant depths is provided.
Other techniques such as atomic force microscopes (AFMs) have also been introduced to provide dimensional analysis of small individual features, but these suffer from not being able to insert the probe tip into the features of interest.
X-ray techniques have also been developed for dimensional analysis, and aspects of some of these are described below.
U.S. Pat. No. 6,680,996 to Yokhin, et al., whose disclosure is incorporated herein by reference, describes a method for testing a surface of a sample. The method includes finding respective first and second critical angles for total external reflection of radiation from an area of the surface at first and second wavelengths.
U.S. Pat. No. 7,110,491 to Mazor, et al., whose disclosure is incorporated herein by reference, describes a method of directing a beam of X-rays to impinge on an area of a periodic feature on a surface of a sample. The X-rays scattered from the surface in a reflection mode are used to detect a spectrum of diffraction in the scattered X-rays as a function of azimuth. The spectrum of diffraction is analyzed in order to determine a dimension of the feature.
U.S. Pat. No. 7,551,719 to Yokhin, et al., whose disclosure is incorporated herein by reference, describes apparatus for analysis of a sample. The apparatus includes a radiation source, which is adapted to direct a first, converging beam of X-rays toward a surface of the sample and to direct a second, collimated beam of the X-rays toward the surface of the sample. A motion assembly moves the radiation source between a first source position, in which the X-rays are directed toward the surface of the sample at a grazing angle, and a second source position, in which the X-rays are directed toward the surface in a vicinity of a Bragg angle of the sample.
Work on X-ray based CD measurements is described by Jones et al., in “Small Angle X-ray Scattering for Sub-100 nm Pattern Characterization,” Applied Physics Letters 83:19 (2003), pages 4059-4061, which is incorporated herein by reference. The authors use transmission-mode small angle X-ray scattering (SAXS) with a synchrotron X-ray source to characterize a series of polymer photoresist gratings formed on a substrate. The X-ray beam passes through the grating and the substrate, and the SAXS pattern is measured using a two-dimensional CCD detector.
Documents incorporated by reference in the present patent application are to be considered an integral part of the application except that, to the extent that any terms are defined in these incorporated documents in a manner that conflicts with definitions made explicitly or implicitly in the present specification, only the definitions in the present specification should be considered.